1. Field of the Invention
The present invention relates to a devices for protecting semiconductor material and a method for protecting semiconductor material.
2. The Prior Art
High-purity semiconductor material is required for the production of solar cells or electronic components, such as for example storage elements or microprocessors. It is therefore desirable to keep the concentration of harmful impurities as low as possible. It is frequently observed that semiconductor material which has already been produced to a high level of purity is contaminated again during the course of further processing to give the desired products. For this reason, expensive purification steps are repeatedly performed in order to maintain the original purity. For example, atoms of foreign metals, which become incorporated into the crystal lattice of the semiconductor material, have an influence on the charge distribution and the life time. These foreign metal atoms can reduce the functioning capacity of the ultimate component or even lead to this component failing. Consequently, contamination of the semiconductor resulting from metallic contaminants is to be avoided. This applies in particular to silicon, which is the most frequently employed semiconductor material in the electronics industry.
High-purity silicon is obtained, for example, by thermal decomposition of silicon compounds which are readily volatile. Hence, these compounds are easy to purify using distillation methods, such as for example the compound trichlorosilane. High-purity silicon is produced using the Siemens process, which is the most commonly employed process. In this process, a mixture of trichlorosilane and hydrogen is guided, in a bell-jar reactor, over thin silicon rods which are heated by the passage of direct current to approximately 1110.degree. C. This produces polycrystalline silicon in the form of rods which have typical diameters of from 70 to 300 mm and lengths of from 500 to 2500 mm. The polycrystalline silicon is used to produce crucible-pulled monocrystals, strips and foils or to manufacture polycrystalline solar cell base material.
To produce these products, it is necessary to melt solid silicon in crucibles. It is desirable to fill the crucible to a high level, and thus to carry out the melting as efficiently as possible. Thus, the above-mentioned polycrystalline silicon rods have to be comminuted prior to melting and then have to be screened. This usually involves contaminating the surface of the semiconductor material. This is because the comminution is carried out using metallic crushing tools, such as jaw or rolling crushers, hammers or chisels on substrates made of materials such as steel or plastic. In addition, the subsequent screening operation usually takes place on screens made from metal or plastic. Thus the silicon is contaminated by metals or carbon from the tools and the substrate during the comminution operation and the screening step. In order to remove this contamination, the fragments have to be subjected prior to melting to a complex and cost-intensive surface purification, for example by etching with HF/HNO.sub.3.
For this reason, silicon substrates and tools made of silicon or with silicon coatings are also used to reduce the contamination during comminution. Screens made of silicon or silicon-coated screens also form part of the prior art in the Screening operation. However, they have the disadvantage that they are damaged or destroyed by the transmission of forces during the comminution operation, such as the hitting with hammers. They can also be damaged during the screening operation, with the result that they have to be replaced. A substrate used both for breaking and for screening on average withstands about 10 to 15 tons of treated material. It is then necessary to replace smashed pieces (approximately 30%), to prevent fragments of the substrate from passing into the material to be used for semiconductor purpose.
Furthermore, these substrates made of silicon have to be disposed of, resulting in further costs. This is because the material is cracked or has been comminuted to undesirable fragment sizes, with the result that it can no longer be used for semi-conductor purpose. The production of such a substrate requires additional separation of silicon, and machining to produce the shaped parts. Also complex purification thereof, is required, for example by etching using HF/HNO.sub.3.